Tap for forming screw-thread-cutting dies



Jain. 17, 1928. 1,656,597

........... ELD

Filed D FE 1 fd I E- E-.- Z

Jan. 17, 1928. 1,656,597

J. D. MERRIFIELD TAP FOR FORMING SCREW THREAD CUTTING DIES Filed Feb. 23, 1923 2 Sheets-Sheet 2 Patented Jan. 17, 1928.

UNITED lSTATES PATENT OFFICE.

JOHN D. MERRIFIELD, OF ERIE, PENNSYLVANIA, ASSIGNOR T0 REED MANUFACTUR- ING COMPANY, OF ERIE, PENNSYLVANIA, A CORPORATION OE PENNSYLVANIA.

TAP FOR yFOIR'IVIING SCREW-THREAD-CUTTING DIES. i

Application led February 23, 1923. Serial No. 620,648.

This invention is designed to simplify the forming of screw thread cutting dies-in the ypreferred form dies which are designed to cut threads by advancing the cut along one slope of the threads. According to my invention I am able to form such dies with the completed throat by the use of a cutting tool specially designed for the purpose. A

, The invention is illustrated in the accompanying drawings wherein the tapis eX- empliied and the means of forming the tap are illustrated as follows Fig. l shows a side elevation partly in section of the tap. 1

F ig. 2 a cross section of the tap cutting the threads of a die.

Fig. 3 a longitudinal section of a portion of the tap in its initial position on a chaser.

F ig. 4 the same view with the tap slightly advanced.

Fig. 5 the tap advanced to the finished cut.

Fig. 6 a plan view showing the finished chaser in position on a thread being cut.

Fig. a plan View of the machine showing the method of forming the tap.

1 marks the tap shank, 2 the tap proper, and 3 the lands formed in the tap. The tap is provided with teeth 4 for cutting the finishing teeth in the body of the die, the teeth 4 being provided with troughs 4il and crests 4b. The tap has also the teeth 5 for cutting the starting teeth of the die in the throat of the die and the teeth 5 have troughs 5a and crests 5b. The tap has also teeth 6 for cutting the entrance teeth of the die in the outer throat of the die and the teeth have the troughs 6a and crests 6b.

Where the cutis along one slope the spiral of the thread, or what might be termed the master spiral, or lead follows along one slope of the teeth and thus is indicated in the drawings by the intersections of the line 7 with the slopes of the teeth. The master spiral marks the crest of the final finished thready which is cut by the die formed by this tap. Tt is the geometric line controlling the lay-out of the thread. The master spiral leads into the trough in this instance where the tapers of the tap shift at 8. Preferably the tap is arranged to form a chaser7 or die, with a throat having a different taper' from the taper of the die. This taper changes at the point 8 and the starting cutters 5 are as shown arranged in the throat.

The cutting teeth have what is termed the ythe final work cut by the die.

front slope 9, this being the slope in front of the crests and rear slope 10.

By forming the flutes in spiral relation the cutting edge is given a more acute angle. Y

This is possible where the entire cut is done along one slope because the negative rake at the opposite slope does not affect the cutting. l In cutting the die thread I prefer tolead it a few thousandths faster or slower depending on which slope is being cut than the spiral of the final thread on the nal work kwhich is cut by the die. The finished die, therefore, has its threads or teeth at a slightly greater inclination than the finished thread on the final work being cut, the iinished thread 15 beingshown in Fig. 6. This is possible where the tool cuts on one slope and the effect on the linished die is to give it a slight relief or clearance in an axial direction back of the cutting edge of the die and to crowd the cutting edge forward by contact of the heel of the teeth on the opposite slope. The amount of thisv variation may be made to correct the lag in lead and Where the die itself is depended upon in feeding the die forward on the work in order t0 put this slight inclination in the die the tap which forms the die has a tooth inclination slightly in advance of the final pitch of the thread which is formed by the die on The die in turn is cut by thetap having a similar characteristic. In other words, an inclination of the master spiral above referred to which follows along the slope of the teeth varies .slightly from the spiral of the final thread.

The method of forming the tap itself is not the subject matter of thisinvention but is described in order that the tap itself kmay `be better understood. A convenient maf chine for this purpose is shown in Fig.y 7. This machine has a head 16 in which is journaled a spindle 17 with a centering device 18 engaging an extension 19 on the tap. The tap is locked with the spindle by `the usual dog 20. A tail-centering piece 21 is shown as operating on the tap.

A cutting tool 22 is mounted in a post 23. The post is carried by a rotating platform 24 on the slide 25. lhe slide is arranged in guides to slide at the angle of the one slope of the tooth, this being the slope which remains constant in the starting teeth. The slide operates against a cam 26, the cam 26 having the earn surtaces and 2G" conforming to the tapers on the tap, it being understood that it it is desired to vary the tapers on the throat that this cam will con-4 torni to it. p p

Any desirable feed forward otx the cutting tool 22 may be provided. `With this method of control the direction ot slide ot' the cutting tool justities, or corrects the cut along the taper. It has been common to feed a tool at the direction of the inclination but the taper heretofore has followed a right angle to the axis and consequently machines ot this type have not corrected for the taper Where the cut is as shown by applicant.

In order to shift the inclination et the starting teeth the tool tor the Inain cut as made is set to rotate on the cutting point of the tool, in other `Words, the plattorin has its axis coincident withthe pointot the cutting tool. A worin gear QT is arranged on the periphery of the platform Q4: and meshes with a Worin 28. rllhe Worin .28 is mounted on a shaft 29. f The shaft 29 is provided with universals 30 and the slip joint 3l. A gear 32 is arranged on the end of the shat't and meshes with a. drire gear 33 of the lathe.

It will be understood that the chasers are arranged in cutting relation as the tap is advanced-into them. In Fig. 2 I have shown the chasers arranged in a solid die. This is held in the ordinary machine, the tap advanced with a lead scren7 following the master spiral which l have before described and the tap is advanced into the chasers to the position forming the proper throat, this position being indicated in Fig. 5.

It Will be noted that at the juncture between the entrance threads G and the starting threads 5 in order to `obviate `an excess cut of the initial thread 5 this thread should be `eased ott troni flute to flute through a part of a revolution of the tap.

That I claim as new is 1. A tap for forning screw thread cutting dies having a plurality ot tapers, one taper being designed t-o forni the teeth in the body ot' the die and one taper to torni the teeth in the throat ot' the die, said tap having its cutting teeth forming the throat of the die with one slope having an inclination varying from the slope of the teeth on the tap preceding them.

2. A tap for forming screw thread cutting dies having its teeth spaced and inclined to forni their cut troni one slope of the die, said tap having a plurality oit tapers,

one taper being designed to form the teeth in the body of the die and one taper to form the teeth in the throat ot the die, said tap having is cutting edge forming the throat et the die With one slope having an inclination Varying from the slope of the teeth on the tap preceding them.

3. A tap tor forming screw thread cutting dies having its teeth spaced and inclined to form their cut from one slope of the die, said tap having a plurality of tapers, one taper being designed to torni the teeth in the body ot the die `and one. taper to torni the the teeth in the throat oi thc die, said tap having its cutting edge forming the throat ot the die having one slope with an inclination varying i'roin the slope ot the preceding teeth, the troughs ot' said cutting teeth in the taper torniing the throat being regularly spaced with relation to the troughs ot the preceding teeth.

Ll. A tap lor forming screw thread cutting dies having a plurallty of tapers, one taper forming the threads of the. body ot the die and the other taper the throat of the die, the teeth formed ou the part of the tap forming the throat finer than the teeth on the taper forming the body olf' the die, said liner teeth with their crests closer together and having pointed troughs and the saine lead as the teeth forming the body of the die.

5. A tap for forming screw thread cutting dies having a plurality ol tapers, one taper cutting the body ot the de `and the other taper cutting the teeth ot the throat of the die, the teeth cutting the throat ot the die forming the intermediate teeth with one slope varying in inclination troni the slope ot the teeth olf the body ot the die., said intermediate teeth preceding entrancc teeth in thc throat, said entrance teeth beng liner and With crests closer together than the teeth forming the body of the die, said entrance teeth haring pointed troughs and the saine inclination as the other teeth ot the tap.

6. A tap ior forming screw thread cutting dies having a plurality of tapers, one taper cutting the. body of the die and the other taper having entraine threads liner and with crests closer together than the teeth cutting the body ot the die but of the saine patch, the. teeth cutting the body oi the die being ot smaller diameter than the teeth 'forming the entrance teeth ot the die.

In testimony whereof l have hereunto set my hand.

JOHN D. MERRIFIELD. 

